(Bill Hale) wrote:

Kyler Laird wrote in message ...
Jim Kaufeld writes:

First verify that your engines are sufficiently grounded. Then
verify it again. Check your alternators while you're there.

There is a copper cable going from the engine to the airframe. It was
recently replaced (on both engines) as part of the STC to install copper cables
to replace the original aluminum cables. BTW, that replacement made a huge
difference starting; before the engines would barely turn over, now they spin
fast!

The ground cable on the engine goes to a ground bus on the firewall; that cable
is copper. A cable runs from the battery to that firewall ground bus. An
aluminum cable goes from there to the insrument panel ground bus.


Kyler gives good advice.

The common mode rejection on most things that measure thermocouple
voltages isn't real high. So anything that can affect engine
grounds, etc can be a problem.

The GEM should be grounded to one of the engines. Doesn't the
1200 have 2 grounds?

The GEM has one harness for each engine. The harness is continuous, that is,
it goes from screw connectors at the probes to the "plug" at the back of the
instrument. The ground and power are at the same point for both harnesses --
the instrument panel circuit breaker and the instrument panel ground bus.

Do you suppose that aluminum cable from the firewall ground bus to the
instrument panel ground bus could be the problem? There is no STC for that to
be replaced by copper ........


Making sure that terminal is staked well would be a start.


We're going to do that tomorrow (Friday), I think. Insight is sending us
another instrument and we're going to bring the airplane in and swap it as soon
as that instrument arrives. It might get delayed until Monday because one of
my partners is going to Sarasota tomorrow, early afternoon and returning on
Sunday.

Bill Hale A&P

Thanks again for the advice. Hopefully we'll track this gremlin down soon.

Jim Kaufeld
Jim Kaufeld